The long-term goal of this project is to further characterize, expand and improve the usefulness of a unique ram model to study the neuroendocrine determinants of sexual orientation. The ram is an exceptional model because variations in male-typical sexual partner preference occur spontaneously, with as many as 8% of the population exhibiting a preference for same-sex mating partners. Sheep have a long period of gestation making them an excellent experimental model for the study of possible links between fetal neuroendocrine programming and adult sexual behavior. Similar to humans, sheep possess a sexual dimorphic preoptic nucleus (SDN) that is larger in males that are attracted to females than in males that are attracted to other males. The ovine SDN (oSDN) develops prior to birth and is enlarged by testosterone exposure during a prenatal critical period that occurs after the external genitals have sexually differentiated. Recently we found that exposure to testosterone prior to the oSDN critical period (Early T) paradoxically reduces the size of the oSDN, suggesting that testosterone exerts opposite effects on masculine oSDN development depending on when it is elevated during gestation. The cellular and molecular mechanisms acting to suppress oSDN development and the impact this has on sexual partner preference are not yet known. Preliminary data show that the hypothalamus-pituitary-gonadal (HPG) axis is sensitive to negative feedback during the oSDN critical period making it possible that Early T could consequently suppress testosterone secretion at the time it is required for oSDN masculinization. Thus, we will test the hypothesis that Early T exposure triggers lasting effects on endocrine feedback and neuronal development that disrupt masculinization of the oSDN later in gestation, producing rams that prefer to mate with other rams but still possess other male-typical neuroendocrine and behavioral traits. My laboratory and collaborators have extensive expertise with the ram model and with the methods needed to elucidate the mechanisms involved in programming the functions of the brain that control sexual attraction. Our specific aims include: 1) Studies in adult rams to determine whether Early T exposure feminizes their sexual partner preferences without affecting other masculine behaviors and neuroendocrine responses; 2) Physiological evaluation of the HPG axis and gene expression to determine how Early T disrupts the masculinization program in lamb fetuses; 3) Cellular and molecular studies of dissociated fetal neuronal cultures to determine the morphological basis for anatomical differences in oSDN size. The project is conceptually innovative because it will test a novel mechanism by which male-typical sexual partner preference and oSDN structure can be demasculinized/ feminized without globally affecting the masculinization program. The project is significant because it will contribute to our understanding of the biological origins of sexual orientation. Finally, advancing the sheep model will provide a valuable animal resource for research bridging the interests of NICHD, NIMH and NINDS.